Various gratings and interferometers have been used to extract the power spectrum of light carried by an optical fiber, including gratings embedded within the core of the fiber itself. As one example, U.S. Pat. No. 5,850,302 discloses an optical waveguide communication system including an optical fiber having a refractive index grating and coupling means selected such that at least a portion of the light is transferred from a guided mode into a radiation mode and is available for utilization by utilization means (e.g., a detector) outside of the waveguide and the coupling means. The optical fiber comprises a chirped and blazed refractive index Bragg grating selected such that at least a portion of the light in the guided mode is transferred into a non-guided mode.
As fiber optic technology continues to grow, the creation of various diffraction gratings within the fibers will become increasingly common. Our co-pending U.S. patent application Ser. No. 09/765,215, incorporated herein by reference, describes a two-dimensional, fiber-delay radiator (FDR). The device is made with optical-fiber taps forming Bragg gratings orientated at 45 degrees to the fiber core, permitting light to emerge directly out of the side of the fiber, linearly polarized. The optical fiber is wound on a cylindrical-like form such that a number of loops of the fiber are available for making a number of taps on each loop. Taps are preferably generated along each loop of the fiber so that a small portion of the light propagating in the fiber will exit sideways from fiber at the taps. A lens system is then used to capture the light from the taps and produce a Fourier Transform of the total distribution of light from all the taps. A video camera then captures this Fourier Transform light and the power spectrum of the light signal is displayed on a monitor.
The preferred construction of the FDR would include taps with “ideal” phase characteristics. To achieve this goal, a phase spatial light modulator may be used to correct and modify the tap phases. Another embodiment uses a coherent reference wave to generate a holographic optical element (or complex spatial light modulator) to correct the tap phases and amplitudes. According to this technique, a coherent reference source and a detector array are used to capture the radiation amplitude pattern of the FDR. Then, with digital processing of the captured pattern, the desired spectral signal properties are obtained.
While it is known to use an ultraviolet (UV) laser interferometer to burn diffraction gratings into optical fibers, it is critically important that the two UV beams of light are caused to converge at precisely the core of the fiber. Accordingly, there is an outstanding need to develop techniques to determine when the two (invisible) UV beams are precisely aligned at the core of the fiber.